This invention is in the field of microfabricated devices. This invention relates generally to the use of microfabricated resonant sensors for measuring cell properties.
The direct dependence of growth rate on cell mass for individual adherent human cells is not understood. Cells undergo multiplication and differentiation within multi-cellular organisms. Understanding how these events are orchestrated by individual cells and cell populations has been of great interest for nearly 50 years. Direct measurements of changes in mammalian cell mass versus growth rate have been among this quest. Such measurements have the potential of elucidating the intrinsic mechanism for coordination between cell cycle and cell growth, and determining whether the growth rate is proportional to the cell size or whether the growth rate is constant over cell size and cell cycle. The linear growth model is based on the assumption that the rate of biosynthesis is limited by the ‘gene dosage’ or the amount of DNA that can initiate the transcription process. On the other hand, the exponential growth model is based on the assumption that the increase of cell mass depends on the amount of ribosomal machinery and cytoplasm. Therefore, as a cell grows larger (or heavier), it has a greater capacity to produce more mass and increase the growth rate. Theoretically, the linear growth can maintain cell size-homeostasis without a size-dependent mechanism, whereas the exponential growth requires a size-dependent mechanism for size-homeostasis.
Recently, great strides have been made towards this goal by interferometric measurements of dry cell mass, population measurements of buoyant mass (analogous to dry cell mass) of suspended cells, and volume measurements of gently synchronized sub-populations of suspended mammalian cells. However, the long-term dependence of growth rate versus mass for individual adherent mammalian cells is unknown. While cell volume can be measured through optical methods, determining cell mass is more complicated based on irregularities of cell shape. Because density and volume can have a disproportionate variance with cell mass and cell-type, determining the true mass of a cell can be influenced by these variations. Irrespective of the cell property being reported (dry mass, buoyant mass, etc.), the methods used to obtain these measures report on a fraction of the whole cells true mass.
MEMS-based resonant mass sensors have been extensively studied as biological and chemical sensors. These sensors measure a shift in the resonance frequency of the structure before and after the target attachment, where the shift can be used to calculate the mass of the target entity. Most of these sensors have utilized a miniaturized cantilever beam structure. The cantilever beam structure is useful for extreme miniaturization due to its simple geometry, and therefore higher mass sensitivity can be achieved. However, as is commonly known, these cantilever beam resonant mass sensors have a spatially non-uniform mass sensitivity. The mass sensitivity is at its maximum when the added mass is placed at the free end of the cantilever and the sensitivity decreases to zero as the added mass gets to the fixed end of the cantilever. If the target entities are much smaller than the sensor and a large number of the target entities are to be attached, then one can assume a uniform mass distribution and use an average mass sensitivity, which can be easily obtained with an analytical solution. However, if only a few or a single target entity is to be attached to the sensor, one cannot assume the uniform distribution of the target mass and one needs to adjust the extracted mass with the mass distribution from optical images of cantilevers, or limit the attachment site to the end of the cantilever. However, these approaches reduce the actual mass sensitivity and make the mass sensor less practical to use.